1. Field of the Invention
The present invention relates to a semiconductor laser in general, and specifically, to a solid laser such as Nd-doped or Yb-doped YAG laser, a Y-doped fiber laser, a semiconductor laser used as an exciting light source for an Er-doped fiber amplified and the like.
2. Background Art
A semiconductor laser is widely used as a light source for optical communication systems and the like. For example, a semiconductor laser having an emission wavelength of 940 nm is formed using an n-type GaAs substrate or the like.
A cross-sectional structure of the above-mentioned semiconductor laser will be described. In this structure, for example, an n-type clad layer, an n-side guide layer, an n-side enhancing layer, an active layer, a p-side enhancing layer, a p-side guide layer, a p-type clad layer, a p-type GaAs contact layer, and a p-electrode are sequentially stacked on an n-type substrate. Furthermore, an n-electrode is formed on the back face of the n-type GaAs substrate.
As a material for the above-described active layer, for example, In0.07Ga0.93As is used. According to “Optical Gain of Interdiffused InGaAs-GaAs and AlGaAs-GaAs Quantum Wells, IEEE J. Quantum. Electoron., Vol. 34, No., 1, pp. 157-165, Jan. 1998, K. S. Chan, E. H. Li, and M. C. Y. Chan”, the band gap energy of In0.07Ga0.93As is 1.319 eV. Hereafter, the above-mentioned document will be described as “K. S. Chan et al.”.
As a material for the above-described n-side guide layer and p-side guide layer, for example, In0.49Ga0.51P is used. According to “Semiconductor Laser, p. 41, K. Iga,” the band gap energy of In0.49Ga0.51P is 1.848 eV. Hereafter, the above-mentioned document will be described as “K. Iga”.
As a material for the above-described n-type clad layer and p-type clad layer, for example. (Al0.3Ga0.7)0.5In0.5P is used. According to “Nondestructive assessment of In0.48(Ga1-xAlx)0.52P films grown on GaAs (001) by low pressure metalorganic chemical vapor deposition, J. Appl. Phys., Vol. 85, No. 7, pp. 3824-3831, Apr. 1999, Z. C. Feng, E. Armour, I. Ferguson, R. A. Stall, T. Holden, L. Malikova, J. Z. Wan, and F. H. Pollak,” the band gap energy of (Al0.3Ga0.7)0.5In0.5P is 1.971 eV. Hereafter, the above-mentioned document will be described as “Z. C. Feng et al.”.
The difference in band gap energies between the active layer and the guide layer of the above-described conventional semiconductor laser is 0.529 eV. The difference in band gap energies between the active layer and the clad layer is 0.652 eV. In this case, the former is 0.81 times the latter. In such a semiconductor laser, as FIG. 9 shows, the rising voltage of voltage-current characteristics (junction voltage) Vj elevates, and the operation voltage elevates. Therefore, when the operation current is predetermined, electrical power injected into the semiconductor laser increases. Then, the ratio of optical output for electrical input (electricity conversion efficiency) is reduced. At this time, since the portion that cannot be taken out as light is converted to heat, the properties and reliability of the semiconductor laser is lowered.
FIG. 10 shows a band diagram of the above-described conventional semiconductor laser in operation. The quasi-Fermi level Ef1 of the conduction band Ec is shown by a dotted line, and the quasi-Fermi level Ef2 of the valence band Ev is shown by a chain line. The locations of the n-type clad layer, the n-side guide layer, the active layer, the p-side guide layer, and the p-type clad layer are denoted by Y1 to Y5, respectively. When the above-described semiconductor laser is in operation, electrons are injected from the n-type clad layer Y1, through the n-side guide layer Y2 and the active layer Y3, to the p-side guide layer Y4. Here, since the n-side guide layer is not doped with an impurity, the density of the electron passing through the n-side guide layer must be equal to the density of the hole passing through the layer. Therefore, when band gap energy difference between the n-side guide layer and the active layer is large, Ef1 in this layer (portion “A”) is largely slanted.
Furthermore, in the above-described operation, holes are injected from the p-type clad layer Y5 through the p-side guide layer Y4 and the active layer Y3, to the n-side guide layer Y2. In this case, by the phenomenon similar to the above-described phenomenon, Ef2 in the p-side guide layer (portion “B”) is largely slanted. When the slopes of Ef1 and Ef2 are generated, the junction voltage Vj rises, and the operation voltage of the semiconductor laser increases.
When the thicknesses of the above-described n-side guide layer and p-side guide layer are less than 100 nm, since electrons and holes can be sufficiently transferred by diffusion or drifting, the reduction of junction voltage Vj can be expected. In this case, however, the light intensity distribution of laser beams is present in the n-type clad layer and p-type clad layer, and the slope efficiency of the semiconductor laser is lowered by the effect of free carrier absorption. In order to avoid this, the thicknesses of the above-described guide layer must be 100 nm or more.